1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the present invention is related to apparatus for separating channel signals with high efficiency and high channel isolation and the method for making the same in compact size.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber.
At a reception end, there needs a device to separate the multiplexed signal into individual signals. For example, in broadband applications, voice signals, data signals and video signals are often multiplexed for transmission over an optical network. When the multiplexed signal arrives in a destination (e.g., a home, a living complex or a business unit), the multiplexed signal needs to be demultiplexed to recover the voice signals, data signals and video signals for proper processing in different devices, for example, the voices signals coupled to telephones, the data signals coupled to computers, and the video signals coupled to display devices. Nevertheless, the practical applications call for devices that can efficiently separate the signals without interfering each other. There have been many efforts in designing such devices.
U.S. Pat. No. 6,493,121 describes a module referred to a transmission and reception module for bidirectional optical message and signal transmission. FIG. 1A duplicates FIG. 2A of the publication. The module composes a fiber pigtail end 0 to transmit downstream signals at wavelengths 1480 nm and 1550 nm signals and receive an upstream signal at wavelength 1300 nm. Two light sensitive components 20 and 30 are used to detect the 1480 nm and 1550 nm signals respectively, and a light emitting component 10 is used for transmitting the 1300 nm signal. To separate the 1480 nm and 1550 nm signals from the 1300 nm signal, each of two 45° oriented thin film filters (TFFs) 22 and 32 is used to couple one of the 1480 mm and 1550 nm signals to a corresponding one of the laser sensitive components 20 and 30.
In reference to FIG. 1A, FIG. 1B shows a principle functional diagram 100 of such implementation. A downstream signal 102, namely a multiplexed signal carries essentially two individual light signals at wavelength 1490 nm and 1550 nm. In the following description, wavelengths 1310 nm and 1490 nm will be used and that may correspond respectively to 1300 nm and 1480 nm used in U.S. Pat. No. 6,493,121 or similar ranges of wavelengths other places. The signal 102 is coupled from a pigtail fiber to a first thin film filter 104 that is configured to reflect a light signal at wavelength 1550 nm. When the signal 102 impinges upon the filter 104, the light signal at wavelength 1550 nm is thus redirected to a (ball) lens 106 that focuses the signal onto a laser sensitive component 108 (e.g., a photodiode). The laser sensitive component 108 converts the light signal to an electronic signal for further processing.
On the other hand, some of the signal 102 transmits through the filter 104 and essentially carries the light signal at wavelength 1490 nm. The 1490 nm signal impinges upon a second thin film filter 110 that is configured to reflect a light signal at wavelength 1490 nm. As a result, the 1490 nm signal is redirected to a (ball) lens 112 that focuses the signal onto a laser sensitive component 114 (e.g., a photodiode). The laser sensitive component 114 converts the light signal to an electronic signal for further processing.
At the same time, an upstream signal is at wavelength 1310 nm and emitted from a laser emitting device 116. The upstream signal is focused by a (ball) lens 118. As the wavelength of the upstream signal differs from the selected wavelength for the filter 104 or 110, the upstream signal thus goes through both of the filters 110 and 104 and is subsequently coupled to the pigtail fiber for transmission.
In principle, the implementation 100 works well in a bidirectional module for multichannel use to separate two or three multiplexed channel signals. However, a careful study of the implementation 100 reveals some problems in practical applications. One of the problems is the isolation between the two separated signals. It will be shown below that the separated signals interfere with each other. In other words, one signal carries a residual or a small portion of another signal. Another one of the problems is the efficiency of the separated signals. Because of the residual of one signal leaking into another, there is a loss to the signal, which can be significant when two signals are different in intensity.
It is well known that a frequency response of a thin film filter depends on an incident angle of a signal impinging upon the filter. When the incident angle is small, the frequency response of the thin film filter is maintained. When the incident angle is large, especially as large as 45° angle used in the implementation 100, the frequency response of the thin film filter is severely degraded. Noticeably, a slope region of the frequency response becomes substantially increased. The slope region, also referred to as a deadband, is the region between a stopband and a passband. In applications of separating channel signals or demultiplexing a multiplexed signal, the deadband is desirably as small as possible such that adjacent channel signals in proximity can still be cleanly separated. FIG. 2A and FIG. 2B show, respectively, two frequency responses of a thin film filter with an incident angle at 0° and 45°. The deadband in FIG. 2A for an incident angle being 0° is approximately 50 nm while the deadband in FIG. 2B for an incident angle being 45° is much greater than 150 nm (it shall be noted that the vertical scales in FIG. 2A and FIG. 2B are different).
For many fiber optic telecommunication applications, such as the fiber to the home (FTTH) application, as illustrated above, the wavelength separation between two channel signals (e.g., the 1550 nm signal and the 1490 nm signal) is often close to 50 nm. Using a uncooled laser for transmission for the purpose of reduced cost, the band separation of the two downstream signals can be as narrow as 30–40 nm. It consequently requires the deadband of a thin film filter no more than 30–50 nm. With an incident angle as large as 45°, it is very difficult to the implementation 100 to achieve a desired separation of such two channel signals. When the channel signals can not be satisfactorily separated, interferences among channel signals could take place. In the implementation 100, the 1550 nm signal and the 1490 nm signal could interfere with each other. It may be worse when a weaker signal is interfered by a small portion of a stronger signal, sufficiently enough to cause distortions or unrecoverable loss of the weaker signal.
In order to reduce the interference among channel signals, high channel signal isolation is desired. A possible improvement over the implementation 100 is to introduce a pair of block filters 120 and 122 as shown in FIG. 3. A block filter can be either an edge filter or a notch filter with transmission characteristics of allowing only the desired channel signal to pass through. Since the block filters 120 and 122 work at nearly 0° incident angle, they can effectively block or isolate other unwanted signals. For example, the block filter 120 has transmission spectral characteristics of passing only the 1550 nm signal and blocking the 1490 nm signal and others. Conversely, the block filter 122 has transmission spectral characteristics of passing only the 1490 nm signal and blocking the 1550 nm signal and others.
Although, the introduction of the blocking filters 120 and 122 can improve the isolation between channel signals, it cannot improve the signal losses caused by the original poor isolation from the filters 104 and 110 (or TFF-1 and TFF-2 of FIG. 3). After the filters 104 or 110, the corresponding reflected signal has incurred certain loss, the addition of the blocking filter 120 or 122 can only isolate the reflected signal from others and may introduce some extra loss to the signal. For some signals, a loss beyond certain percentages to its intensity could damage the signal significantly.
FIG. 4A shows some notations about a thin film filter 400 with an incident angle at 45°. The thin film filter 400 may correspond to the filter 104 or 110 in FIG. 1B or TFF-1 and TFF-2 of FIG. 3. It is assumed that a multiplexed signal 401 includes two channel signals at two different wavelengths λ1 and λ2. To simplify the description, the two channel signals are simply expressed as λ1 and λ2. The multiplexed signal is coupled to the filter 400 which is configured to reflect λ1 and transmit λ2. Because the frequency response of the filter 400 depends on the incident angle of the signal, at 45°, the reflected signal 402 contains not only the λ1 signal but also some portion of the λ2 signal.
For better understanding, a notation /xy is used, wherein/means intensity, x indicates a wavelength and y is either “m” or “n”, indicating “majority” or “minority”. Thus the reflected signal 402 may be expressed as /λ1m+/λ2n, which means it includes a majority of the λ1 signal and a minority or small amount of the λ2 signal. Similarly, the transmitted signal 404 contains not only the λ2 signal but also a small amount of the λ1 signal and may be expressed as /λ1n+/λ2m. If it is assumed that the filter has an negligible absorption, then the following expressions can be true:/λ1m+/λ1n=/λ1 and /λ2m+/λ2n=/λ2.where /λ1 means the total intensity of the λ1 signal, and /λ2 means the total intensity of the λ2 signal.
In reference to FIG. 2A and FIG. 2B, the relative intensities of the λ1 signal and the λ2 signal in the transmitted signal 404 and the reflected signal 402 are listed in a table 450 shown in FIG. 4B. It may be observed from the table 450 that shows comparisons between incident angles 0° and 45°, nearly 91% of the λ2 signal transmits through the filter 400 at 45° while nearly 95.5% of the λ2 signal transmits through the filter 400 at 0°. However, the reflected signal includes only 68% of the λ1 signal reflected by the filter 400 at 45° while the reflected signal includes 99% of the λ2 signal reflected by the filter 400 at 0°. The signal loss evidently is much higher at 45° than at 0°.
In conclusion, the problems in the implementation 100 can be significant in some applications. Accordingly, there is a great need for techniques for an optical module that separate channel signals with high efficiency and high channel isolation. The devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.